Mark Miller's sealer / unsealer code

gistfile1.js

function makeBrand() {
    var key = {};
    class Envelope(payload) {
        protected const contents = payload;
        function open(k) {
            if (k === key) { return this.contents; }
        }
    }
    return {
        sealer: function(payload) { return new Envelope(payload); },
        unsealer: function(env :Envelope) { return env.open(key); }
    };
}